1. Field of the Disclosure
The present disclosure is generally related to radio frequency receivers, and more particularly, to correction of received signals in receiver systems.
2. Description of the Related Art
Modern communications systems transmit and receive information by modulating a radio frequency (RF) carrier signal with a data signal. The data signal can be at a much lower frequency than the RF signal. Such systems can then demodulate the RF signal to recover the data signal.
Many RF receiver systems use some form of heterodyning to convert a received RF signal to a lower frequency signal (sometimes called an intermediate frequency signal), which may be easier to filter. Generally, heterodyning refers to a process of mixing (or multiplying) a first signal with a second signal having a frequency that is close to that of the first signal. In this instance, the RF signal is often multiplied with a local oscillator signal (LO signal). Mixing the two signals results in two signals, a first signal having a frequency equal to the sum of the RF frequency and the LO frequency, and a second signal having a frequency equal to the difference between the RF and the LO frequencies. The first frequency is higher than the RF or LO frequency, and is usually filtered readily using a simple low-pass filter. The difference frequency is the intermediate frequency (IF), which can be manipulated using fixed frequency filters.
Unfortunately, typical heterodyne-based systems are susceptible to a phenomenon referred to as imaging. Imaging refers a signaling phenomenon where two different RF signals are translated to the same intermediate frequency, thereby causing interference. In general, a desired RF frequency fRF differs from a given LO signal frequency fLO by the IF frequency fIF. A desired radio frequency may lie either above or below the LO signal frequency. However, due to its symmetric properties, heterodyning systems sometimes select any RF signal differing from fLO by fIF, regardless of whether the RF signal lies above or below fLO. For example, if a desired RF signal has a frequency of 1.01 GHz and the LO signal has a frequency of 1.00 GHz, the two signals can be mixed to produce an IF signal having an IF frequency of 10 MHz. However, if there is a second RF signal with a frequency of about 990 MHz, the receiver will mix both the 1.01 GHz and the 990 MHz signals to the same frequency of 10 MHz, thereby causing interference with the desired signal. The image frequency can be, for example, the frequency corresponding to the sum of fLO and fIF.
To prevent interference with the desired RF signal, some communication systems use quadrature receiver architectures for splitting the desired RF signal into two paths and for mixing each path with a respective function of a local oscillator signal, where the respective functions have a ninety-degree phase difference. One of the paths is typically referred to as an in-phase (I) signal path, and the other path is typically referred to as a quadrature (Q) signal path.
If the phase relationship of the Q signal is exactly 90 degrees out of phase with the I signal, and if the I path and the Q path circuits are identical in terms of amplitude and phase, then the image signal is perfectly rejected from the desired signal. Fortunately, Quadrature IF mixing allows for cancellation of image signals without expensive and bulky rejection filters. However, if any non-idealities exist in the signals (imperfect 90 degree phase difference) or if the I and Q paths are imbalanced or mismatched (phase, amplitude, and so on), then the gain and phase of the I/Q path circuit will cause the image signal to leak into the desired signal, resulting in imperfect image cancellation.
To improve image rejection or cancellation, some receivers utilize a calibration tone to calibrate the receiver to account for any gain and/or phase imbalances between the two paths. For example, some systems may use a calibration tone as an input to a quadrature mixer during a calibration mode. A residual image signal can then be measured to derive an IQ mismatch correction factor to be applied to the I signal path and/or to the Q signal path to adjust the gain and phase of one path to improve image rejection.
Mismatches between the I and Q signal paths and image rejection in general are very important in RF transmission systems with a plurality of transmitters and receivers and with receivers wherein fRF±fLO also lies in the band of interest, because the image channel can be much stronger than the desired channel and the image channel cannot be filtered easily because it is in-band. Various calibration schemes have been proposed to compensate the I/Q gain/phase mismatch based on achievable native I/Q matching in analog circuitry; however, most of such calibration schemes assume that the gain/phase mismatches are frequency independent across the entire desired tuning range of the local oscillator.
However, in modern communication systems, wide channel bandwidths are often used because they provide higher data rates than narrow channel bandwidths, and wider channels can more easily exhibit frequency dependent gain/phase mismatches. For example, in television broadcasting, these wider bandwidths are evident because channel widths are commonly 6 MHz to 8 MHz.